Thin multiconstituent films can be found in a great variety of technological products, from optical lenses to integrated circuits, and a plethora of methods for producing such films exist. Since this application is concerned with a deposition method carried out in a vacuum environment, my discussion will be confined to such vacuum methods.
Among the vacuum deposition methods those carried out at low temperatures are of increasing importance in, inter alia, the semiconductor industry. For instance, in silicon very large scale integration (VLSI) technology, it is desirable to maintain processing temperatures below about 600.degree. C. to avoid substrate warpage, dopant diffusion, and other yield- or device characteristic-affecting problems. Similarly, in the compound semiconductor device field (e.g., GaAs devices), low processing temperatures are required, due to the low surface incongruent evaporation temperatures and the high chemical reactivity of these materials. In general, it can be said that low temperature operability is a very important characteristic of a material deposition process for use in the semiconductor industry or in industries using similar technologies.
However, many of the standard prior art methods for forming thin films of multiconstituent materials on a substrate typically require relatively high processing temperatures. Methods in this category are thermal oxidation or nitridation of silicon, which produce SiO.sub.2 and Si.sub.3 N.sub.4 by consuming substrate material in the presence of oxygen or nitrogen, respectively, and chemical vapor deposition (CVD), which produces multiconstituent material by means of thermally induced reactions among appropriate gaseous reactants in contact with a substrate surface.
Among prior art deposition methods that can be practiced at relatively low substrate temperature is sputtering (including reactive sputtering), evaporation (including activated reactive evaporation), plasma deposition, plasma assisted CVD, molecular beam deposition, and ion beam deposition. Some of these techniques will now be briefly reviewed.
The "activated reactive evaporation process", a plasma-enhanced deposition process, uses metal or alloy vapors from an evaporation source together with a gas that is capable of chemically reacting with the vapor and that is injected into a reaction region inside a vacuum chamber. The reaction between vapor and gas atoms may be encouraged to go to completion by activating and/or ionizing both the metal and gas atoms in the vapor phase. This is typically accomplished through acceleration of secondary electrons formed in a thin plasma sheet above the metal vapor source, acceleration typically being by means of an electrode placed between deposition substrate and evaporator. The thus accelerated electrons create a plasma-filled region between the electrode and the evaporator in which the metal and gas atoms are ionized or activated, leading to an increased probability of reaction between the two species. See, for instance, R. R. Bunshah, Thin Solid Films, Vol. 80, pp. 255-261, (1981). Because of the proximity of the plasma region to the substrate and of the presence of relatively energetic electrons, the method typically subjects the substrate to ion and/or electron bombardment.
Another prior art method is plasma deposition as exemplified by Pat. G.B. No. 2,076,587A, issued to Nippon Telegraph and Telephone Public Corporation, for "Plasma Deposition Apparatus", published Dec. 2, 1981. The patent discloses apparatus having a separate plasma formation chamber and specimen chamber, with a plasma stream passing through the plasma extracting orifice of the plasma formation chamber into the specimen chamber, where the stream is accelerated by the effect of a divergent magnetic field. In the plasma chamber, gases introduced therein are activated and/or ionized by microwaves. The thus activated accelerated particles impinge on the substrate and react thereon to form a deposit film. All chemical constituents of the film are present in the plasma chamber and are extracted therefrom. For instance, in order to form a silicon nitride film, both silane gas (SiH.sub.4) and ammonia gas (NH.sub.3) are introduced into the plasma chamber.
It will be noted that the above discussed low temperature deposition methods use plasma-created ionized or excited species to drive the desired reaction. It is a common feature of low temperature methods that energy is supplied to the reaction region by nonthermal means, typically through the presence therein of an excited or ionized particle species.
In addition to operability at low temperature there are several other desirable characteristics of a multiconstituent material deposition process useful in semiconductor device processing and similar applications. For instance, it is desirable, and will probablay become increasingly more important in VLSI device manufacture, that such a process does not cause any substantial substrate damage, i.e., cause unwanted modifications of the substrate surface and/or adjacent substrate material, since such damage can adversely affect device characteristics. Such damage can, for instance, be caused by bombardment with relatively energetic charged particles. Several prior art processes typically do expose the substrate to such particles. These processes include deposition in a neutral or ionized gaseous medium, plasma deposition, and the activated reactive evaporation process.
A further relevant process characteristic is its contamination susceptibility. In particular, a process in which particles, produced by the interaction of the reactive medium, e.g., plasma, with a foreign substance, e.g., a reactor wall, have free access to the deposition region, is typically more likely to produce contaminated deposits than is a process in which such access is restricted or eliminated.
The final process characteristic I wish to mention is the control of deposit stoichiometry afforded by a multiconstituent material deposition process. Some prior art processes either permit only limited control of deposit compostion (e.g., CVD), are difficult to control due to the process' sensitivity to some processing parameter (e.g., molecular beam depostion's typical sensitivity to source temperature), or inherently tend to produce nonstoichiometric deposits.
Prior art methods for depositing multiconstituent material on a substrate that are applicable to semiconductor device manufacture and to similar technologies typically do not possess all of the above discussed desirable characteristics. Although at least some of these methods perform adequately at current levels of silicon integrated device technology, it is expected that further device miniaturization and improvements in device characteristics may require improvements with regard to these deposition process characteristics. A process that can form high quality multiconstituent material at very low substrate temperature, that can be practiced such as not to cause any significant substrate damage and to be essentially contamination free, and that allows close and relatively easy control of deposit stoichiometry thus is of substantial technological interest.